The present invention relates to a coating composition which forms a cross-linked hardened film on the substrate to which it is applied, the film exhibiting high abrasion resistance, cloud prevention properties, surface smoothness and dyeability and having a permanently high antistatic performance when irradiated, cured and cross-linked with active energy rays.
Synthetic resin moldings made of polymethyl methacrylate, polycarbonate or polyallyldiglycol carbonate have various commercial advantages in that they are not only lighter in the weight and substantially more shockproof than a corresponding glass product, but they are also cheaper in price and easier to fabricate than glass products. Accordingly, these materials have several practical applications in replacements for their glass counterparts, such as organic glass plates, light fixture covers, optical lenses, eyeglass lenses, sunglass lenses, reflectors and mirrors, diaplay panels, signboards, commercial advertizing displays, name plates, dust cover cases as well as automobile parts.
However, conventional synthetic resin moldings are particularly definient in surface abrasion resistance properties and are susceptible to damage on the surfaces, by contact with other objects, shocks and scratches during the transportation and storage or while in use. These surface scratches not only detract from the appearance of the article but also affect the properties of the article; for instance, scratches in the molded synthetic article when used as an optical surface in an optical device. Particularly, when the end uses of the coated moldings are for cameras, magnifying glasses, fashion glasses, sun glasses, correction lenses, window glass plates, decorative cases, display cases, dust covers, watch lenses, reflectors and mirrors, a surface scratch or imperfection will remarkably reduce the commercial value of the moldings and will make them unsaleable within a short period of time. Improvements in the abrasion resistance of the surface of such molded plastic articles are required.
A polyfunctional (meth)acrylate monomer has already been described as exhibiting high cross-linking hardening on polymerization when irradiated with active energy rays and is effective to improve the abrasion resistance of the surface of a synthetic resin molding and such fact has been disclosed, for example, in U.S. patent application Ser. No. 878,830 filed on Feb. 17, 1978, now U.S. Pat. No. 4,199,421.
While this has been an advancement in the art, there still remain problems to be resolved. For instance, the film formed on the surface of a synthetic resin molding is a cross-linked hardened film, it is difficult to diffuse the dye into the film so that the film may be dyed if so required. Also, when the film is subjected to moisture, it readily becomes clouded. Further, when friction is applied to the article, such as during cleaning, the film will generate static electrocity causing dust to be readily deposited on it. This detracts from the visual impression of the article, particularly when dust is deposited on difficult to clean surfaces such as the inside surface of a watch crystal. Accordingly, the value of the molding will be remarkably reduced, the range of end use applications is limited and cleaning or maintenance difficulties are encountered in practice.
A proposal has been made for improving the dyeability and cloud prevention properties of a coating which is a method in which either a compound having several dyeing coupling agents or cites, or a hydrophilic compound is mixed in a coating material composition. However, when such compound is included in the coating composition, the abrasion resistance will be reduced. As for preventing the generation of static electricity, there are procedures known in which an antistatic agent is mixed with the coating or a method in which a monomer itself having antistaticity properties is copolymerized with the coating monomer. The addition of an antistatic-type material is a convenient production method but only the antistatic agent present on the surface of the coating will develop the desired effect. Thus, in order to obtain sufficient preventing properties on the one hand, a high concentration of the antistatic agent is required, while on the other hand the abrasion resistance and appearance of the article will be reduced. Further, as the antistatic agent present on the surface will be easily dissipated by washing the surface or with friction and the coating will depreciate in its static properties. If the antistatic agent is present in the coating matrix, it may gradually seep out, however, this seepage tends to make the surface sticky and translucent, i.e., no longer optically transparent. Further, any durability againt washing or the like is only temporary and is not permanent. With respect to the copolymerizing type of antistatic agent, no monomer other than in the surface layer will develop the effect, therefore a large amount of the monomer must be added and mixed and as a result the abrasion resistance and appearance will be reduced accordingly.